The present invention relates to an ion transfer membrane assembly for electrochemical processes, and a method of construction thereof, and an electrochemical cell utilizing the membrane assembly. The membrane assembly is of the same general type illustrated and described in U.S. Pat. No. 4,242,193, including a plurality of layers of capillary material. Like the membrane in said patent 4,242,193, the membrane according to the present invention comprises a metal anion permeable, non-ionic, semi-permeable, non-permselective membrane, having no membrane potential and capable of providing passage of anions thereacross over extended operation without destructive swelling, clogging, chemical reaction, or consumption thereof.
While the membrane disclosed in U.S. Pat. No. 4,242,193 has been found eminently suitable for a wide variety of chemical processes, where the gradient of concentration is high, the membrane disclosed in U.S. Pat. No. 4,242,193 has some limitations. These limitations are overcome according to the present invention, the present membrane being capable of utilization even where the gradient of concentration is extremely high.
According to the present invention, an improved (for many applications) type of capillary material is utilized in a membrane construction generally like that in U.S. Pat. No. 4,242,193. In said patent, the types of capillary materials specifically disclosed were porous materials having natural capillarity, such as paper, polyester non-woven webs, etc. The pore size in such natural capillary materials are not completely controllable, however, and it has been found according to the present invention that where a capillary material is utilized that has very precisely controllable capillary size, it is possible to construct a membrane that is utilizable even where the gradient of concentration is extremely high.
A membrane assembly according to the present invention provides capillaries of controlled size by providing as a membrane a plurality of layers of capillary material each being formed from a corrosion-resistant, relatively soft, film having surface manifestations formed thereon defining capillaries extending from one side thereof across the width thereof to the other side thereof. Thermoplastic films, such as polyvinyl chloride, polyethylene, polyester, polypropylene, and polyurethane films, are particularly suited for a membrane construction, and the film may be embossed, frosted, etched, scribed, or formed with a lumpy surface to provide the necessary surface manifestations. The layers are compressed together with a compression means to control the water permeability of the membrane and to maintain its structural integrity.
The layers of thermoplastic film capillary material may be interspersed with layers of other types of material. For instance, a plurality of layers of corrosion-resistant, springy, porous capillary material, such as webs of woven or non-woven synthetic fiber fabric (e.g., polyester non-woven webs) may be interspersed with the capillary material film layers, and/or a plurality of layers of smooth-surfaced film may be interspersed with a capillary material film layers. While in many environments a composite membrane of a variety of layers of different types of material is desirable, in some environments the membrane consists of said plurality of capillary material film layers, adjacent film layers having non-interlocking surface manifestations.
In its broadest aspect, the method of construction of an ion transfer membrane assembly for electrochemical processes is produced by forming a plurality of capillary-like manifestations in a normally non-capillary material, and assembling the non-capillary material with capillary-like manifestations into a membrane having a desired water permeability, with capillaries extending from one side of the membrane to the other. According to the preferred manner of practicing the present invention, the capillary-like manifestations in a non-capillary material are created by forming a plurality of sheets of thermoplastic film with surface manifestations (as by embossing, frosting, etching, etc.), the surface manifestations extending across the width thereof. The sheets are then stacked and a compressive force is applied to them to control the water permeability. Again, interspersed with the capillary material films may be layers of corrosion-resistant, springy, porous capillary material and/or layers of smooth-surfaced film. By precisely controlling the formation (size, pattern, etc.) of the capillaries associated with the normally non-capillary material, and by precisely controlling the compressive force applied to the membrane, an ion transfer membrane having virtually any desired water permeability may be formed.
An electrochemical cell constructed according to the present invention includes an anode disposed in an anode chamber, a cathode disposed in a cathode chamber, and a membrane assembly disposed between the anode and cathode chambers. The membrane assembly comprises a metal anion permeable, non-ionic, semi-permeable, non-permselective membrane having no membrane potential and capable of providing passage of anions across over extended operation (i.e., hundred of hours if not years) without destructive swelling, clogging, chemical reaction, or consumption thereof. The membrane comprises a non-capillary material with controlled capillaries introduced therein and extending from the anode chamber to the cathode chamber. Preferably, the membrane comprises a plurality of layers of thermoplastic film having surface manifestations formed thereon which define the capillaries.
It is the primary object of the present invention to provide an ion transfer membrane having precisely controlled capillaries extending thereacross, and capable of operating effectively with extremely high gradients of concentration. This and other objects of the present invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.